It would help to talk a little bit about the soil in the pot. What is it? It's got an odd, light-colored appearance I don't think I've ever seen before. Does that pot have a drain hole?

If your plant is always inside, there's no wind blowing it to cause the resistance it needs to fight against to develop strength. The support also allows it to be weak and lean.

Ming Aralia is 1. sun loving, 2. slow-growing, 3. does better in smaller pots, 4. detest wet soil.
If you feel the container is too large, perhaps repotting, 'possibly spring,' will speed growth, thicken trunks.

Which direction/window is your Ming nearest and what size was its old pot and new?

Another possiblity is dividing one trunk and potting solo. Remaining trunks can stay together in the same pot or smaller, if you decide to reduce container size.

I bought this Ming last May because of its thick trunk. No waiting, lol.

Notice trunk size? It arrived in a 3" container, fully rooted..I repotted in a pot one size larger.
Unless it dies, Ming will stay in its 4" pot a year or two, depending on root development.

You said you give your Ming Aralia a drink every Sunday.
I'm not telling you when or how to water, but it's important soil dries, especially in winter when days are grey. When soil is dry, water thoroughly.

Don't expect much growth this time of year. Additional, artificial lighting might help, but Ming's are naturally slow-growers.
That's one reason many growers buy larger specimens. Ever see Ming's in store-fronts or Chinese restaurants.

Anyway, a bright window and/or additional light might help thicken trunks...Good luck, Toni

Your plant really doesn't like small pots, and small pots SLOW growth and thickening of stems. You'll get the best growth in plants that offer plenty of room for roots to run, as long as you use a soil that doesn't support significant volumes of perched water. How much perched water your soil supports is the pivotal issue that determines how large your pot can or should be - not how big the plant or root mass is now. I often plant very small plants in very large pots when rapid growth is what I'm looking for. I can do this because the soils I use hold little to no perched water. In case you're not familiar with that term, 'perched water' is the water in that soggy layer of soil at the bottom of the pot that refuses to drain from the pot under ordinary circumstances and raises cain with root health and root function in a very large % of the soils we commonly use.

Al

The author talks about preferring a soil with a lot of peat in it (which holds water - the perched water mentioned above - for a long time, which can cause roots to rot,) which would explain the author's preference toward smaller pots. Either way, whether with a smaller pot, or a soil that doesn't hold this water to begin with, the goal is to prevent sogginess.

The fallacy of the smaller pot of peaty soil is that the soil is indeed soggy until it dries to just moist, the speed of which would be determined by the size of the plant, the amount of roots in proportion to the pot, temperature, humidity, the health of the roots, and the plants' thirst at the time. A smaller pot with less soil (but still with a lot of peat) will be in this condition for a shorter time than a larger pot of mostly peat, so there may be no damage. But I agree that the plant will grow more quickly if the roots have room to also grow quickly in not-soggy soil.

Nothing wrong with deciding you want a plant to inhabit a particular pot for aesthetic reasons, but if the roots don't have room to grow, one can't also expect to have a rapidly-growing plant in a too-small pot.

Thanks, Al.

Ironically, I Googled Polyscias soil and soil pH, each authors' answer differs, 'a tad' in opinions.
Some say ph should be neutral, while others state slightly acidic. Whom to believe?

As for pot size..Pot size depends on height/roots. Since Azegger's Polyscias is large, naturally a 4" container wouldn't be suitable.

Ming Aralia can be grown in standard or bonsai pots.
But, either way, roots should fill the container before repotting.

If grown as a bonsai, roots need trimming, but if one wants a taller specimen roots will do w/o cutting.

Google Ming Aralia..There's many beautiful specimens to be seen. Bonsai and standards.

Azegger.. Lower leaves drop naturally as Ming Aralia matures. Polyscias are known as small trees to many.

Which part of the foliage is falling?
Have you moved your Ming Aralia to lower light by chance? Toni

It sounds like you are treating the plant the way it likes it....I don't trust though a north window for the lots of light this plant craves.
Perhaps, if you could move it to a more westerly, or southern window and pull it back from the window--as governed by the season--in winter, it can be given more direct light though.
The pot seems to me to be a type of ceramic...it does appear to have a drainage part of it...hopefully, when you water you do so to drainage...then let it dry down between waterings. Watering on a schedule like you do invites all kinds of problems. You should treat the plant according to its needs....not yours. Try testing the soil by poking your finger down into it to see if it needs water. If it feels damp, let it go another couple days and test again.
Make sure your soil drains---and don't let the water sit in the saucer more than 10 - 15 minutes before disposing.